In a rapidly developing information society, highly integrated semiconductor devices capable of achieving rapid data transfer rates are required so as to process data more quickly. In order to improve the data transfer rates, unit cells are integrated into a chip with a high degree of integration. For example, the design rules of metal wires have been rapidly decreasing in size. The metal wire is generally formed using copper (Cu). The metal wire including copper may has a relatively low resistivity. Further, when the metal wire is formed using copper, voids in the metal wire, which may occur due to an electromigration phenomenon, may be suppressed. However, patterning a copper layer including copper using an etching process may be complicated because chemical reactions of copper atoms with chemicals or gas are very complex. Further, the copper layer tends to be easily oxidized. Particularly, when the copper layer is exposed to air, the copper layer is easily oxidized to increase the electrical resistivity and the stress of the metal wire.
In order to solve the above-mentioned problems, a damascene process has been employed to form the metal wire including copper. FIG. 1 is a cross-sectional view illustrating a metal wire formed by a conventional damascene process.
Referring to FIG. 1, an insulation layer 10 is partially etched to form a trench 15 at a surface of the insulation layer 10. The trench 15 is filled with copper to form a copper layer (not shown) in the trench. The copper layer is then planarized by a chemical mechanical polishing (CMP) process until an upper face of the insulation layer 10 is exposed to form a metal wire 20 in the trench 15. However, while the copper layer is planarized by the CMP process, the copper layer may include a surface portion having a groove, which is called a dishing phenomenon, because the copper layer has a polishing rate greater than that of the insulation layer 10. The dishing phenomenon may occur severely when a width (W) of the metal wiring 10 becomes greater. Thus, the sheet resistance of the metal wire 20 including copper may increase such that a semiconductor device including the metal wire 20 may have deteriorated reliability.